This invention relates to an electrophotographic photoreceptor for electrophotography.
Until now, an electrophotographic photoreceptor has been prepared from inorganic materials such as CdS, ZnO, Se, Se-Te, or amorphous silicon, or organic materials such as poly-N-vinylcarbazole (PVCZ) or trinitrofluorenon (TNF). However, these conventional photoconductive materials have present various difficulties in manufacturing the subject product. Consequently, these materials have been selectively used in accordance with the intended object, with some lack in the desired properties of a photosensitive system.
For example, Se and CdS are harmful to the human body, demanding particular care in manufacturing, from the point of ensuring safety. Therefore, these materials are accompanied with the drawbacks that the manufacturing phase involves a complicated process, resulting in a high manufacturing cost and high recovery costs due to the required recovery of Se. Moreover, the Se and Se-Te series have as low a crystallization temperature as 65.degree. C. Therefore, when copying is repeated, difficulties arise with respect to the photoconductive property, for example, in residual potential. Consequently, the Se and Se-Te series have a short effective life and reduced practicability.
Moreover, ZnO easily undergoes oxygen reduction, and is noticeably affected by exposure to the atmosphere, and has low reliability in application.
Further, organic photoconductive materials such as PVC and TNF are suspected to be carcinogens. These materials present difficulties from the point of view of safety to the human body, and, what is worse, are handicapped by low thermal stability, abrasion resistance and a short effective life, as is characteristic of organic materials.
On the other hand, amorphous silicon (hereinafter abbreviated as "a-Si") has recently attracted wide attention as a photoelectric converting material, and has been successfully applied for use in a solar cell, thin film transistor and image sensor. Description will now be made of the application of a-Si as the photoconductive material of an electrophotographic photosensitive member (Japanese patent disclosure Sho No. 59-12448). Not only a-Si is harmless and need not be recovered, but also it has a higher spectroscopic sensitivity in the region of visible rays than other materials. Further, it has a great resistance to abrasion and impact due to its significant surface hardness.
Research has been done on a-Si as a photosensitive member for electrophotography, based on the Carlson process. In this case, a photosensitive material having high dark resistance and photosensitivity is required. It is difficult, however, to provide a single layer photosensitive element which can satisfy both requirements. Hence, the conventional practice is to provide a barrier layer between the photoconductive layer and conductive support, and to deposit a surface charge-sustaining layer on the photoconductive layer, so that the resultant laminate structure may meet the abovementioned requirements.
Description may now be made of a-Si. Generally, this material is manufactured by the glow discharge decomposition process involving the application of silane series gas. In this case, hydrogen is carried into the a-Si layer. Electrical and optical properties noticeably vary with the content of hydrogen. Namely, the greater the quantity of hydrogen carried into the a-Si layer, the more enlarged the optical band gap, and consequently the resistance of the a-Si layer is raised. Since the a-Si layer is more reduced in sensitivity to the light rays having long wavelengths, it is difficult to practically utilize a laser beam printer equipped with, for example, a semiconductor laser device. When the a-Si layer contains much hydrogen, the greater part of the layer is sometimes occupied, for example, by a structure consisting of (SiH.sub.2).sub.n bonded with SiH.sub.2. In this case, voids are noticeably generated, and silicon dangling bonds are increasingly produced. This event reduces the photoconductive property of the a-Si layer so much that the a-Si layer fails to serve a an electrophotographic photosensitive member. If, conversely, smaller quantities are taken into the a-Si layer, the optical band gap is reduced and decreases in resistance, but increases in the sensitivity to light rays having long wavelengths. The conventional a-Si layer, manufactured by the customary film-forming process, has a drawback. If its hydrogen content decreases, it tends to be coupled with silicon dangling bonds, resulting in a decrease in the content of hydrogen, which is desired to minimize said coupling. Therefore, there is the risk that generated carriers drop in transmission speed and have a reduced life, leading to the deterioration of the photoconductivity property of the a-Si layer, thereby rendering said a-Si layer unusable as an electrophotographic photosensitive member.
Description may now be made of the process of elevating the sensitivity of said a-Si layer to light ray having long wavelengths. This process comprises the steps of mixing a silane-series gas with germane GeH.sub.4, applying glow discharge decomposition, and producing a layer having a narrow optical band gap. Generally, however, silane-series gas and GeH.sub.4 have different optimum substrate temperatures, resulting in the occurrence of structural defects in the resultant layer and the failure to provide a satisfactory photoconductive property. The spent gas of GeH.sub.4, if oxidized, will be converted into a noxious gas. Therefore, the treatment of the spent GeH.sub.4 gas involves complicated steps. Consequently, the above-mentioned process involving the mixture of silane series gas and germane gas (GeH.sub.4) lacks practicability.